16: Studies for Neurologic Evaluation

INTRODUCTION

Continued improvements in neuroimaging and neuromonitoring have added insight into the developing brain and helped the clinician to identify infants at risk for poor neurologic outcome. However, available techniques continue to be limited in their ability to predict neurodevelopmental outcomes accurately. Moreover, given the enormous plasticity of the neonate's brain, even significant detectable defects may result in “normal” neurodevelopmental outcomes. Nevertheless, imaging and monitoring modalities hold future promise in assisting clinicians to better identify patients at risk for neurodevelopmental sequelae.

I. NEUROIMAGING

1. Ultrasonography

   1. Definition. Using the bone window of a fontanelle, sound waves are directed into the brain and reflected according to the echodensity of the underlying structures. The reflected waves are used to create 2- and 3-dimensional images.

   2. Indication. Ultrasonography is preferred for identification and observation of germinal matrix/intraventricular hemorrhage and hydrocephalus and is valuable in detecting midline structural abnormalities, hypoxic-ischemic injury, subdural and posterior fossa hemorrhage, ventriculitis, tumors, cysts, and vascular abnormalities. Ultrasonography of the developing cingulate sulcus has been suggested to reflect gestational age (see sample studies in Chapter 11).

   3. Method. A transducer is placed over the anterior fontanelle, and images are obtained in coronal and parasagittal planes. The posterior fontanelle is the preferred acoustic window for the imaging of the infratentorium, including brainstem and cerebellum. Advantages include high resolution, convenience (performed at the bedside), safety (no sedation, contrast material, or radiation), noninvasiveness, and low cost. Disadvantages include the lack of visualization of nonmidline structures, especially in the parietal regions, and the lack of differentiation between gray and white matter.

   4. Results. The integrity of the following structures may be evaluated with ultrasonography: all 4 ventricles, the choroid plexus, caudate nuclei, thalamus, septum pellucidum, and corpus callosum.

2. Doppler ultrasonography

   1. Definition. Doppler ultrasonography also uses a bone window to direct sound waves into the brain. Moving objects (eg, red blood cells) reflect sound waves with a shift in frequency (Doppler shift) that is proportional to their speed. These changes are measured and expressed as the pulsatility index and resistance index (RI). The angle of the probe in relation to the flow affects the Doppler shift and requires exact standards for serial measurements.

   2. Indication. Knowing the cross-section of the vessel (area), Doppler ultrasonography can provide information on cerebral blood flow (CBF) and resistance. CBF (cm³/time) = CBF velocity (cm/time) × Area (cm²) Doppler ultrasonography is of clinical value in states of cessation of CBF (eg, brain death or cerebrovascular occlusion), states of altered vascular resistance (eg, hypoxic-ischemic encephalopathy, hydrocephalus, or arteriovenous [AV] malformation), and ductal steal syndrome.

   3. Method. Combined with conventional ultrasonography to identify the blood vessel, Doppler ultrasonography produces a color image indicating flow (red, toward the transducer; blue, away from the transducer). CBF velocity is measured as the area under the curve of velocity waveforms. Small body weight and low gestational ages negatively influence the success rate in visualizing intracranial vasculature. Contrast-enhanced ultrasonography with injection of gas-filled microbubbles ...